Originally Posted by eponymous_coward

Won't happen before single loyalty plan, maybe even SOC. "I'm sorry, but you're evicted from your upgraded F seat because we now have an 8F cabin, and no F upgrades either, pay or sit in Y. Oh, please pay some money to get any seat in the Y cabin other than 28E, even though you're a 75K, but we'll give some Elevate Silvers those nicer seats. Thanks for being a loyal Alaska customer!"

There's any number of other ways they can do this, such as "break up the flight and fly the plane PVR-SFO-PDX, have all the SEA pax board their different SEA-bound plane at T2, transfer luggage."